In olefin epoxidation an olefin is reacted with oxygen in the presence of a silver-based catalyst to form the olefin epoxide. The olefin oxide may be reacted with water, an alcohol or an amine to form a 1,2-diol, a 1,2-diol ether or an alkanolamine. Thus, 1,2-diols, 1,2-diol ethers and alkanolamines may be produced in a multi-step process comprising olefin epoxidation and converting the formed olefin oxide with water, an alcohol or an amine.
Conventional silver-based catalysts have provided the olefin oxide notoriously in a low selectivity. For example, when using a conventional catalyst, the selectivity towards ethylene oxide, expressed as a fraction of the ethylene converted, does not reach values above the 6/7 or 85.7 mole-% limit. Therefore, this limit has long been considered to be the theoretically maximal selectivity of this reaction, based on the stoichiometry of the reaction equation7C2H4+6O2=>6C2H4O+2CO2+2H2O,cf. Kirk-Othmer's Encyclopedia of Chemical Technology, 3rd ed., Vol. 9, 1980, p. 445.
The catalysts are also subject to an aging-related performance decline during normal operation. The aging manifests itself by a reduction in the activity of the catalyst. Usually, when a reduction in activity of the catalyst is manifest, the reaction temperature is increased in order to compensate for the reduction in activity. The reaction temperature may be increased until it becomes undesirably high, at which point in time the catalyst is deemed to be at the end of its lifetime and would need to be exchanged.
Generally, the commercially applied olefin epoxidation catalysts are shaped catalysts which comprise silver deposited on a support. They are prepared by a method which involves impregnating or coating the shaped support with a solution comprising a silver component. The support is commonly prepared by moulding a dough comprising the support material or a precursor thereof into shaped particles and drying the particles at a high temperature of, for example, at least 1000° C. Numerous patent publications disclose examples of such catalyst preparation.
Over the years much effort has been devoted to improving olefin epoxidation catalysts in their performance, for example in respect of their initial activity and selectivity, and in respect of their stability performance, that is their resistance against the aging-related performance decline. Solutions have been found in improved compositions of the catalysts, and, in other instances, solutions have been found in improved processes of preparing the catalysts.
Modern silver-based catalysts are more selective towards olefin oxide production. When using the modern catalysts in the epoxidation of ethylene the selectivity towards ethylene oxide can reach values above the 6/7 or 85.7 mole-% limit referred to hereinbefore. Such high-selectivity catalysts may comprise as their active components silver, and one or more high-selectivity dopants, such as components comprising rhenium, tungsten, chromium or molybdenum. High-selectivity catalysts are disclosed, for example, in U.S. Pat. No. 4,761,394 and U.S. Pat. No. 4,766,105.
In respect of improved processes of preparing the catalysts, for example, U.S. Pat. No. 6,153,556 shows that utilizing a support prepared by mixing α-alumina particles with a silicon compound, an organic binder, and a metal or compound selected from the Groups Ib and IIb in the Periodic Table of elements leads to catalysts which have improved initial performance properties.
WO 2004/030813 shows that forming shaped particles from a paste comprising an alkaline earth metal carbonate support material and a silver bonding additive improves the mechanical properties of the resulting support.
In some instances, epoxidation catalysts are formed from a dough comprising silver in addition to the support material. In particular, with respect to such epoxidation catalysts, it may be desirable to improve the attrition resistance of the catalysts. Within commercial processes, friction or rubbing occurs between the catalysts themselves or between the catalyst and equipment surfaces. This friction or rubbing may occur during catalyst manufacturing, catalyst shipping, epoxidation reactor loading, or other reactor processes. These forces can cause the catalyst to breakdown into smaller particles called fines. This physical breakdown of the catalyst is known as attrition.
Attrition occurring during the loading of the catalyst into the epoxidation reactor can cause dusting problems which results in a loss of valuable catalyst. The difficulty associated with attrition with respect to the epoxidation process is that the fines can be driven away from the reaction zone, resulting in 1) excessive developments of the reaction in the separators or other locations within the oxidation process and 2) creating problems in the recovery systems. The loss of catalyst reduces the productivity of the catalyst bed effecting overall process efficiency and increasing operating costs. Thus, it would be highly desirable to improve the attrition resistance of catalysts.
It also goes without saying that—despite the many improvements already seen—it remains highly desirable to improve the performance, in respect of one or more of activity, selectivity and stability, of olefin epoxidation catalysts formed from a dough comprising silver in addition to the support material.